The Resize page on the management UI lists a "Linode 3072" plan. This isn't described on www.linode.com nor can I find a reference to it in the library. Could someone from Linode tell me what are the features and cost of this plan?

It should be a specced between the plans it's listed as on the resize page, and (from what i can tell) the plan is made available so you have a middle ground between the 2048 and the 4096.
As for specs, it should be in the region of 3GB RAM, same 8 vCPU's, 144GB Storage, and 6TB Transfer all for the low price of $60/mo.

IIRC, that one should be deprecated. There was a linode 1536, so the last doubling would take those to 3072, but I thought only those who had them could keep them and they weren't available for new linodes anymore.

Not available as NEW linodes, but you can resize to them

It's probably because of the nature of cloud hosting; they've got a bunch of legacy customers on 3 gig linodes, and those host servers aren't fully utilized. So while they don't want to sell new 3 gig linodes (because that has implications about availability, and they don't want to deploy new 3gb linode hosts), they can allow people to resize to them to try to fill the gaps. That's my guess, anyhow.